1. Field of the Invention
This invention is in the field of nuclear medicine and more particularly in the field of radionuclide emission imaging.
2. Description of the Prior Art
These are currently two general modes which use ionizing radiation for diagnosing and delineating diseased tissue within the human body. These are the transmission and emission modes. In the former, penetrating radiation, such as X-rays. is emitted from a source external to a body and transmitted therethrough. In the latter, a radionuclide contained within a body emits penetrating radiation, such as gamma rays, which similarly pass through the body.
In a conventional transmission system, X-rays diverge from a source, penetrate the body to be examined and impinge on a sheet of photosensitive material such as film. The density at each point on the film is representative of the total transmission on the X-rays along a pathway through the body. Increased or diminished attenuation in a small volume along this column is difficult to detect because of the superimposition of the signals from all elementary volumes along a path.
Recently, breakthroughs have occurred with transmission systems which permit three-dimensional image reconstruction of the tissue density distribution of an organ. This makes it possible to image internal structures in three dimensions and to visually display such information. These systems employ mathematical techniques for reconstruction of an image in three dimensions by combining images from two-dimensional scans from different angles, usually referred to as transaxial scans.
One three-dimensional X-ray reconstruction apparatus which has been widely used is known as a "CAT" (computerized axial tomographic) scanner. This scanner is described in a number of United States patents issued in the name of Godfrey Hounsfield, including: U.S. Pat. Nos. 3,778,614; 3,866,047; 3,867,634; 3,881,110; 3,919,552; 3,924,131; and 3,932,757.
In a typical brain scan with the CAT scanner, a patient is positoned with his head cradled in a square gantry. An X-ray tube scans along one side of the gantry coupled to a detector on the opposing parallel side. The entire rectangular structure rotates about an axis through the head. With each 1.degree. rotation of the rectangular gantry, the scanner (X-ray tube and coupled detector) makes a single traverse. Along each scan, a number, n, of intensity measurements of a transmitted narrow beam of X-rays are made in contiguous increments and the gantry is ultimately rotated through 180.degree.. Every brain site on a planar section through the brain is thus intersected by the beam n times. Computer reconstruction of the attenuation coefficients of volume elements in the planar section of the brain traversed by the X-rays is accomplished by well known algorithms for the reconstruction of a transaxial section from parallel projections. The scanner actually measures two contiguous planes at a time. Repetitions, following incremental translations of the patient, produces four scans or 8 sections which are usually sufficient to image most adult heads. The family of transectional images constitutes a three-dimensional image of attenuation coefficients for the brain and surrounding tissues.
In typical two-dimensional emission imaging applications, a source of penetrating radiation is administered to the patient. Typically, this consists of a radiopharmaceutical capable of gamma-ray emission. In the so called "rectilinear scanners", gamma-rays emitted and directed along columns defined by lead collimators are recorded by sodium iodide scintillation detectors. Through rectilinear scanning, the detected intensities are transformed into a two-dimensional image which may be displayed on a cathode ray tube or a hard copy such as a film.
So called "scintillation cameras" are capable of imaging an entire organ, such as the brain, without detector motion. In such cameras, a set of parallel lead collimated holes defines a set of columnar trajectories generally perpendicular to the camera face. The detector is constructed to be position sensitive, i.e., the trajectory from whence a gamma-ray originated is identified. This permits direct reconstruction of a two-dimensional projected image by an intensity format on a cathode ray tube and a hard copy such as a film.
While two-dimensional radionuclide emission techniques are widely used, it has been recognized that they suffer from significant drawbacks. Thus, a three-dimensional radionuclide distribution in the interior of an object under examination appears with its details from front to back superimposed. Consequently, the resulting two-dimensional image is often difficult to interpret in that concentrations of activity within small volumes are often not identifiable or adequately pinpointed.
Emission apparatus suitable for three-dimensional image reconstruction has been developed. In one apparatus developed by Kuhl et al., each of four discrete detectors scans rectilinearily along the peripheries of a square gantry. Each detector is collimated so that it can only see gamma-rays directed in a column along its axis. Thus, in each scan, four sets (views) of transaxial parallel projected data are collected for reconstruction. The gantry rotates in increments about an axis perpendicular to the plane of the projections to collect a complete set of views needed for reconstruction, which is done using computerized iterative techniques. See Kuhl, D. E., Edwards, R. Q., Alavi, A., Reivich, M. and Rothenberg, H.; "Radionuclide Computerized Tomography for Brain Study"; Workshop on Reconstruction Tomography in Diagnostic Radiography and Nuclear Medicine, San Juan, Puerto Rico, April 1975. Later embodiments of the Kuhl apparatus include eight detectors positioned contiguously along each scan path. Each detector consists of a sodium iodide scintillation crystal individually collimated and coupled to separate light pipes and photomultipliers.
Additionally, a few proposals have been made for three-dimensional reconstruction using a scintillation camera for detecting gamma-rays from single photoemitters. See Budinger, T. F. and Gullberg, G. T., "Three Dimensional Reconstruction in Nuclear Medicine Emission Imaging," IEEE Trans 21, (3), 202(1974); and Keyes, J. W., "Clinical Application of Transaxial Tomography in Nuclear Medicine. Image Processing for 2-D and 3-D Reconstruction from Projection," OSA, August, 1975. Each of these uses existing two-dimensional scintillation cameras that are arranged to make measurements in parallel projection geometries similar to Kuhl's. These radionuclide imaging systems have significant limitations. Among these are limited resolution because parallel projections and the planar shape of the detector limits the resolution obtainable by projection of radiation for transaxial reconstruction. The planar shape of the scintillation camera also limits the sensitivity obtainable by projection of radiation for transaxial reconstruction. Additionally, position analysis of the scintillation event as usually employed does not optimize the resolution and generally produces distortions and results in non-uniformities in sensitivity which are known to be as high as 15% in practice.
Regardless of the method of measuring emitted penetrating radiation, the reconstructed transaxial section images will be erroneous unless attenuation by the radionuclide-containing-body is accounted for. A method of correction has been proposed by Kuhl. See Kuhl, D. E., Edwards, R. W., Abass, A., Reivich, M., and Rothenberg, H.: Radionuclide Computerized Tomography for Brain Study (In) Ter-Pogassian, M. M., Phelps, M. E., Brownell, G. L., Ed: Workshop on Reconstruction Tomography in Diagnostic Radiology and Nuclear Medicine, San Juan, Puerto Rico, 17-19 April, 1975, University Park Press, New York, 1976. In this work an attenuation correction is applied after reconstruction by correction factors experimentally obtained from a homogeneous cylinder containing a uniform radionuclide distribution. Another method of correction has been proposed by T. Budinger and G. T. Gullberg. See Budinger, T. F. and Gullberg, G. T.: Three-Dimensional Reconstruction in Nuclear Medicine by Iterative Least Square and Fourier Transform Techniques. Donner Laboratory, 1974, LBL - 2146. In this case, the projections are approximately corrected for attenuation by geometrical combination of opposing data and from body geometry attenuation corrections before reconstruction. Neither of these correction techniques is entirely satisfactory.
Therefore, an object of the present invention is to provide a method and apparatus for mapping the emission concentration of a radionuclide within a body whereby differences in concentration in small elements in a planar slice can be determined.
Another object is to provide improved sensitivity for transaxial radionuclide sectional imaging.
Another object is to provide improved resolution for transaxial radionuclide sectional imaging.
Another object is to provide a method of position analysis of scintillation camera system whereby resolution and uniformity of detection is improved.
Another object is to provide an improved method of analysis of the data to correct for attenuation.
Another object is to provide a method for direct reconstruction of transaxial images from divergent projections which is capable of analyzing the data "on-line" with its collection.
Another object is to provide a method and apparatus for mapping a radionuclide distribution within a body whereby the distribution is portrayed by cylindrical projections on a cylindrical surface.